This proposal concerns molecular events involved in light responses of the ferredoxin gene Fed 1. These responses are quite different from those of other well-studied light regulated genes with respect to the speed and reversibility of light effects. We have recently shown that cis-acting elements responsible are located within the transcribed portion of the gene. This arrangement is unprecedented in plant systems and strongly suggests that post-transcriptional events affecting Fed 1 mRNA stability has not yet received much attention in plant systems, and the Fed 1 gene offers an opportunity to develop a major model system. The first major objective of the present proposal is to further define the relative importance of transcriptional and post-transcriptional events at different stages of plant development. Transcription and mRNa degradation rates will be measured in transgenic plants (or plant cells) carrying various chimeric Fed 1 genes. Both green plants and etiolated seedings will be examined, since diurnal modulation of Fed 1 mRNA levels in mature leaves may occur by a different mechanism than the first induction of gene expression in seedlings. The second objective is to further define the cis-acting sequences required for light effects on Fed 1 mRNA accumulation. As noted above, these sequences are located within the single Fed 1 exon. Effective element(s) will be defined using in vitro mutagenesis in combination with functional assays ranging from transgenic plants to protoplast electroporation with synthetic mRNA. Elements conferring light responses in green leaves or cultured cells will also be compared to those involved in the induction of Fed 1 in dark-grown seedlings. Finally, defined light regulatory elements will be used to begin studies on the biochemical basis of the light response and on factors which may participate in specific interactions with Fed 1 mRNA.